698 REPORT— 1903. 



Department op Physiology. 



1. A Plbysiological Theory to Explain the Winter -whitening of Birds and 

 Mammals in Snoivy Countries, and the most Striking Points in the 

 Distribution of White in Vertebrates generally. By Captain G. E. H. 

 Barrett-Hamilton.^ 



The subject of the wioter-whitening of animals, though of much interest to 

 zoologists, is very imperfectly understood. Most writers are satisfied to believe 

 that the colour change originated somehow under the action of natural selection 

 for the protective .purposes of adaptation to environment. 



The author finds, however, that the change has a deep physiological significance. 

 There is, for instance, in mammals a definite sequence in which the various parts 

 of the body whiten. This sequence, on the whole, corresponds to the summer 

 accumulation of fat in the panniculus adiposus. Thus the bellj', where peripheral 

 fat is thickest, is permanently white, and the rump, often the next thickest area 

 of fat accumulation, is usually the first part to whiten in winter. 



Many northern mammals and birds not usually regarded as of this category are 

 lighter in winter than in summer. The wliite assumed in the former season cor- 

 responds to the fat-tracts, and they may be therefore regarded as subject to tiie 

 same process. 



In the northern summer most animals accumulate fat, always in a definite 

 manner as regards the regions where it is deposited. This fat is indicative of 

 deficient oxidisation and sluggish metabolism, and the process of its accumulation 

 is therefore one of atrophy. The fat-accumulation and atrophy is most marked in 

 autumn, at which season metabolism is lowest. Under the onset of winter cold the 

 atrophj' may extend to the hairs. Their pigment (as observed by Metchnikotf) is 

 then removed, always, however, first in those parts where peripheral fat is thickest, 

 and atrophy therefore greatest. Should there be a change of coat at this time the 

 new hairs are influenced by the same conditions. In very cold countries they 

 come up white all over the animal ; in more temperate regions the parts only whei'e 

 fat is thickest are white. 



Although a pigmented hair can thus undergo atrophy and loss of pigment, the 

 author knows of no case where the colour is replaced. Animals once whitened 

 remain so until the spring moult. 



These facts apply broadly to birds and mammals, but the variable hare and 

 stoat are those which have been studied especially. 



The same law is responsible for much of the distribution of the wliite colour 

 throughout the vertebrate phylum, wherein the connection between the white 

 colour and tlie peripheral fat-tracts (thus indicating local atrophy) may be widely 

 traced. Thus domestic animals, nearly all of which are prized most for their 

 power of accumulating fat, exhibit a .strong tendency to the development of white 

 patches. In both tlaese and in wild animals the belly, where occurs the principal 

 fat-tract, is the most frequently white part ; next follow the rump, neck, and parts 

 of tlie limbs and of the head. 



Marked exceptions are no doubt frequently due to unusual arrangements of the 

 panniculus adiposus. Thus in the badger, a representative of a family in which the 

 back is usually whiter than the belly, a correspondingly exceptional arrangement 

 of the fat-tracts occurs. 



The white of the head — the ' blaze ' of horses and the facial stripes of the 

 badger, for instance — afi'ects regions, not of fat-accumulation, but where the skin 

 immediately overlies bone and membrane (frontals, nasals, and zygomatic arches), 

 Avhich thus seem to produce an atrophy similar to tliat caused by underlying fat. 



In many animals the hair-atrophy assumes the form, not of whitening, but of 

 baldness. Marine mammals are hairless in proportion to their fatness ; fattening 

 cattle lose their hair, while the baldness of man corresponds in position to the 

 ' blaze ' of horses, and the bare buttocks of monkeys to the white rumps of other 

 mammals. 



' Will appear in the Proceedings of the Royal Irish Acaiemi/, 



